The present invention relates to the field of metallized substrates for communications applications and, more particularly, to a means of tuning thin film structures such as stripline filters, using automated low-power laser trimming.
Metallization on non-conductive substrates such as ceramics is well known in the art for such circuit components as stripline filters. Such filters typically include a configuration or pattern of conductive elements on one surface of the ceramic plate, these elements interacting with a relatively large area of ground plane on the opposing substrate surface. For electrical reasons, the ground plane is required to be a very low conductivity layer, typically including a heavy (one mil) layer of plated copper. Filters or other such components may require removal of a portion of the conductive ground plane for tuning or other adjustment. Such adjustments have been made by abrasion; i.e., mechanically removing small portions of the ground plane in the appropriate areas, using a diamond grinding wheel or sand trimmer. It will be obvious that such a method does not lend itself to automated trimming procedures, where the circuitry is monitored as it is tuned and the monitoring equipment controls an extremely precise laser beam. Thick film resistors on substrates can be trimmed precisely by low-power (1.8 kw input power) laser beams, but that power is not sufficient to trim a heavily plated ground plane.